The efficiency of sails and sail systems depend upon the relation of two forces namely lift and drag. All attempts to increase the performance of sails have been aimed at increasing the lift force to drag force ratio. By increasing the lift through better aerodynamic shape, or decreasing the drag by streamlining better sails have been developed. In addition to the drag associated with the airfoil section of the sail there is an additional drag from the supporting structure that will typically reduce the efficiency of the entire sail system. Supporting structures include masts, booms, spars and cables which all cause additional drag and tend to lower the lift to drag ratio of the sail system.
Various attempts have been made to lower the drag imparted to the sail system by the supporting structures. Masts, normally attached to the leading edge, cause a turbulent wake that disturbs the airflow to the sail and reduces aerodynamic lift and causes drag. One method used on many high performance sailboats has been to streamline the mast and allow rotation so that the narrowest section is across the impinging air flow. The extreme of this approach is called the wing mast, a rigid mast section, having up to 25% of the total sail area, has a flexible fabric sail cloth attached at its trailing edge to form the remaining portion of the sail. The increased efficiency is a result of having thickness in the airfoil section much the same as on an airplane wing, the intersection between the wing mast and the fabric portion however is not a smooth junction and thus some drag is still inherent. Other improvements extending from the wing mast approach has been the development of total rigid airfoil sails, (ex. Barkla U.S. Pat. No. 2,804,038 and Smith U.S. Pat. No. 3,295,487).
Another approach has been to have a freely hanging sail away from the mast, as are conventional jibs and genoas, and eliminate the mast turbulent effect on the airfoil, (eg. Darby "Popular Science" August 1965 pages 138-141 and Jamieson U.S. Pat. No. 4,044,702). The mast drag still remains although drag reduction can be accomplished by covering the mast with a rotatable streamlined fairing.
These developments have succeeded to reduce the drag effect of the supporting structures however they all have inherent disadvantages. The wing mast and solid wing structure is the airfoil and is very heavy and thus causes additional drag on the vehicle or craft being propelled. The triangular freely hanging sails are relatively high for sails having areas equal to sails with masts at the forward edge because of the inward curves of the edge tension and support elements, hence the masts to support such must be even higher than the sails they suspend and therefore add a considerable amount of drag to the sail system.
It is my invention to "eliminate" the drag associated with the sail support structure and to increase the airfoil lift by enclosing the structure within a double fabric high lift airfoil section.